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Same approach. Took me about 3 min to reach this though. I also get 13. So I guess A it is then.

This problem looks harder than it really is. Essentially what walker and I did here is realize that all we need to care about is the number of 5's from 99-199. we don't include the evens so just start counting out the odds with factors of 5:

105, 115, 125, etc...

105 -> 5*21 115-> 5*23 125-> 5*25 etc...

We see that there should be a 5^13 as a factor of the product of the odd integers from 99 to 199.